Conventionally wellbores, and Christmas trees associated with wellbores, have been sealed with plugs having three basic parts: an anchoring system, a sealing element and a setting system.
The first stage in setting a conventional plug is anchoring the plug in the wellbore. Anchoring systems for conventional wellhead plugs use a set of locking dogs, which engage a recessed profile in the wellbore or tree, or use a set of slips which “bite” the casing to hold the plug in place.
The seal is then set using a linear action setting mechanism to create a linear displacement to deform the seal element. The force required to create the seal is then locked in using a linear locking mechanism. In safety critical wellbore applications, for example sub sea trees, the seal is generally a metal-to-metal seal formed by swaging a metal ring element into the bore or onto a no-go shoulder.
To provide a seal capable of withstanding well pressures, the required setting force needs to be as high as the maximum force generated by the well pressure.
In recent years a number of high pressure, high temperature, high flow rate wells have been completed which have highlighted shortcomings in conventional designs of well bore plugs and tree plugs. For example, swaged seals can dislodge when exposed to the high pressure, temperature and vibration cycles of these wells, and the jarring action used to set the seal can damage the plug or the surrounding environment.
Additionally, linear locking mechanisms have a degree of backlash which in a high temperature, pressure and vibration cycle environment can lead to failure.
A further disadvantage of conventional plugs is that the expansion achievable from the metal seal element is not sufficient to permit the plug to be run into the wellbore with adequate clearance between the plug and the wellbore to prevent a build-up of pressure in front of the plug, resisting the placement of the plug. This can be a particular problem when a number of plugs are to be located in series in a conduit, as a hydraulic lock can be formed between plugs.
It is an object of the present invention to obviate or mitigate at least one of the aforementioned disadvantages.